One for customers and investors, and one for employees if the company feels it is needed and wants to spend the money on two web sites.
Yes, they can - it might be done if they have a lot of info to put out there or separate divisions that have different information. It really wouldn't make sense though unless they have separate business to run. Microsoft is a pretty big company with one website.
There are mirror sites that can be operating too - fake sites that are meant to defraud people. That's happened in banking, that's why financial institutions have set up extra security measures for login.
Yes,they can even if you want to attract more people to your business with a different domaine name that is searched allot for.
They can have as many as they like but each one will be different.
